Polylactic acid obtained by ring opening polymerization of a lactide is well known as a biodegradable polymer which is susceptible to hydrolysis and also degradation by microorganisms. Conventional processes for producing polylactic acid are roughly divided into a method consisting of direct dehydrating polycondensation of lactic acid and a method consisting of dehydrating cyclization of lactic acid to obtain a lactide and ring opening polymerization of the lactide.
The former direct polycondensation method meets difficulty in obtaining a polymer having a molecular weight higher than 4,000 (see C. H. Halten, Lactic Acid, p. 226, Verlag Chemie (1971)). Even with manipulation added to reaction conditions for increasing the molecular weight, the highest molecular weight reached is about 20,000 as described in JP-B-2-52930 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Therefore, the latter ring opening polymerization method has been used for producing higher molecular weight polymers as taught in JP-B-56-14688.
The ring opening polymerization method has conventionally been carried out in a batch system, in which a solid lactide is charged in a stirred tank reactor, melted by heating and ring opening polymerized in the presence of a catalyst. Polylactic acid obtained by ring opening polymerization of a lactide increases its viscosity to a very high value of 10,000 poise to hundreds of thousands of poises with an increase in average molecular weight. Partly for this reason and partly because the melting point of polylactic acid is 160.degree. C. or higher, it is preferable to keep the reaction system at a high temperature to reduce the viscosity. However, the problem is that polylactic acid or a polylactic acid copolymer easily undergoes reduction in molecular weight by heat.
Even in a closed reactor, degradation initiates at around 180.degree. C., and molecular weight reduction is accelerated at a high temperature of 250.degree. C. or higher. It has therefore been difficult, with a conventional stirred tank reactor, to continuously produce polylactic acid or a lactide copolymer having a high viscosity in a narrow temperature distribution range while preventing runaway of the reaction due to the heat of reaction.
Application of a method for continuously polymerizing monomers using stirred flow reactors connected in series to production of a lactide copolymer is proposed in JP-A-5-93050 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Monomers copolymerizable with a lactide by this method include epoxy compounds, such as propylene oxide, intermolecular cyclic esters, such as glycollide, lactones, such as .epsilon.-caprolactone; and cyclic carbonate monomers, such as trimethylene carbonate.
Copolymers obtained by a lactide and the abovementioned comonomer usually have lower melting points and lower glass transition points than a lactic acid homopolymer. The transparency of the copolymers is also lower than that of polylactic acid. Further, the copolymerization shows a lower rate of reaction than the homopolymerization of a lactide, whereby reducing the productivity in spite of continuous production. The apparatus disclosed is composed of two stirred flow tanks connected in series. With this structure, however, raw materials continuously fed are incorporated into a product through a bypass in actual operation even if the retention time is extended, only to provide an opaque polymer or a polymer having insufficient physical properties.
Additionally, the reaction disclosed in JP-A-5-93050 supra is limited to homopolymerization of a lactide or copolymerization of a lactide and a copolymerizable monomer. Cases are unknown, in which a lactide and a polymer are continuously reacted in an apparatus composed of three or more stirred flow reactors connected in series.